1. Field of the Invention
The present invention relates a high strength spring steel that has excellent hydrogen embrittlement resistance, particularly to a high strength spring steel wherein hydrogen embrittlement, season crack and delayed fracture, that cause troubles for spring steels having tensile strength of 1860 MPa or higher, are suppressed.
2. Description of the Related Art
Chemical compositions of spring steels, that are used to make high strength springs such as valve spring of automobile engine and suspension spring, are specified in JIS G 3565-3567 and JIS G 4801. The springs are manufactured by drawing a hot rolled steel of the specified composition until the wire diameter is reduced to a predetermined size, applying oil tempering treatment and processing the steel wire into a spring (cold formed coil spring), or by drawing a rolled steel, forming the steel wire into spring by heating, followed by quenching and tempering (hot formed coil spring).
As automobiles are increasingly designed to be lighter in weight, springs used in the automobiles are required to have higher strength and some springs have achieved tensile strength of 1800 MPa or higher. However, increasing strength of springs causes concern over the problem of so-called delayed fracture, in which the spring suddenly cracks after a long period of use.
To counter such a problem, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-183302 describes a method of improving corrosion resistance by adding alloy elements such as Cr, V, Ni, Cu, B and/or Nb to the basic components and improving delayed destruction resistance by is making the crystal grains smaller. Japanese Patent Publication No. 3064672 describes a method of improving toughness and corrosion resistance of the steel that has been subjected to quenching and tempering by adding Ni, Cr, Cu, V to the basic components, thereby improving the fatigue setting resistance and the hydrogen embrittlement resistance.
Japanese Unexamined Patent Publication (Kokai) No. 2001-288539 describes a method of improving hydrogen embrittlement resistance by containing at least one of oxide, carbide and nitride that contain one or more kind selected from among V, Mo, Ti, Nb and Zr as a hydrogen trap site and composite precipitate of two or more kinds of these compounds. Specifically, hydrogen embrittlement resistance is improved by controlling the mean grain size of the precipitate within a range from 0.05 μm to 1.0 μm, and controlling the mean grain distance within a range from 3 to 30 times the mean grain size.
However, the alloy elements used in these technologies are expensive and it is difficult to provide a high strength spring steel that has high level of delayed destruction resistance at a low price. There is also such a problem that it is difficult to recycle a spring that contains much contents of the alloy elements described above.
Japanese Unexamined Patent Publication (Kokai) No. 2004-143482 describes that hydrogen embrittlement resistance has been improved by controlling the structure by a method that does not require the addition of the alloy elements described above. Specifically, hydrogen embrittlement resistance of high strength spring steel wire is improved by making a structure that is constituted mainly from martensite or bainite where prior austenite crystal grains are made smaller, and limiting the number of coarse undissolved carbide grains. However, there is a limitation to the improvement in the hydrogen storing capacity by controlling the form of precipitation, and it is difficult to achieve higher hydrogen embrittlement resistance by this method.